For air-cooled, heat-producing electronic equipment requiring two or more racks arranged in rows on a raised floor, such as in a large computer installation, the cooling system may cause air to flow across the electronics in any of the three coordinate directions. Let X be the direction along a row, Y be perpendicular to the rows, and Z be upward. For systems in which the air flows across the electronics in the X direction, air-flow plenums typically must exist between each pair of racks.
FIG. 1 illustrates a prior-art arrangement of such plenums, 22A, 24A, 22B, 24B, interspersed between a row of heat-producing racks 20A, 20B, 20C. Although the figure depicts just three racks, in general the row may contain an arbitrary number of such racks. In the prior-art, hot plenums 22A, 22B and cold plenums 24A, 24B alternate along the row, as shown in FIG. 1, such that racks 20A and 20B share the cold-air supply provided by cold plenum 24A, while racks 20B and 20C share the hot-air exhaust provided by hot plenum 22B. This sharing scheme continues along the row as suggested by the air-flow arrows at the extreme right and left of the figure. As shown in FIG. 1, the hot plenums 22A and 22B are closed at their bottoms, while the cold plenums 24A and 24B are closed at their tops. As shown by the arrows 26A, 26B, 26C and 26D, air flows upward from the raised floor 28, through holes in floor tiles, into the cold plenums 24A and 24B, and is drawn into the adjoining racks 20A, 20B and 20C as indicated by the arrows 27. After flowing across the electronics in a rack (generally in the form of circuit cards, as more fully described with respect to FIG. 3A), the air emerges into a hot plenum 22A or 22B, as shown by the arrows 29, where it is joined by air emerging likewise from the rack on the opposite side of the hot plenum. This air flows upward to the exhaust, generally to be re-circulated after cooling.
The present invention is based on a discovery of a problem with this prior-art scheme; namely, that it does not efficiently use the space occupied by the plenums, because the constant width of the plenums is not matched to the variable volumetric flow rate of air that the various horizontal cross sections of the plenums must carry.
Specifically, the volumetric flow rate of air in each cold plenum is greatest at the bottom, because the bottom cross section must carry the full complement of air to feed the entire pair of adjoining racks. As distance z from the floor 28 increases, the volumetric flow rate in the cold plenum gradually diminishes as air flows into the racks. Finally, near the top of the cold plenum, the volumetric flow rate is nearly zero, inasmuch as nearly all the air has flowed into the racks.
This inefficiency of the prior art is depicted graphically in FIG. 1 by the upwardly decreasing thickness of the arrows 26A, 26B, 26C and 26D in the cold plenums 24A and 24B. The arrows' thickness decreases to emphasize that a cold plenum carries a lower and lower volumetric flow rate of air as z increases, inasmuch as its air is drawn into the adjoining racks as indicated by arrows 27. If the distribution of electronics is uniform top to bottom, the diminution of air flow in the cold plenums is roughly linear in z. In particular, the upward air flow is zero at the top of the cold plenum. In spite of this variable flow situation, the cross-sectional area of the prior-art cold plenum is the same top to bottom. Thus space is wasted near the top of the cold plenum.
Likewise, the inefficiency of the prior art is depicted in FIG. 1 by the upwardly increasing thickness of the arrows 32A, 32B, 32C and 32D in the hot plenums 22A and 22B. The arrows' thickness increases to emphasize that a hot plenum carries a higher and higher flow rate of air as z increases, inasmuch as it collects air along the entire height of the adjoining racks as indicated by arrows 29. Finally, at the top, the volumetric flow rate in the hot plenums 22A and 22B is a maximum, because the top cross section must carry the accumulated complement of air that has emerged from the entire height of the adjoining racks. In spite of this variable flow situation, the cross sectional area of the prior-art hot plenum is the same top to bottom. Thus space is wasted near the bottom of the hot plenum.